Electric amplifier



y 29, 1934- G. BEERS 1,9 0,723

ELECTRIC AMPLIFIER Filed Aug. 13, 1927 3 Sheets-Sheet l INVENTORGray/21.5 m

ATTORNEY May 2 1934- e. L. BEERS ELECTRIC AMPLIFIER Filed Aug. 13, 1927s SheetS-Sheet 3 INVENTOR George LBeers.

ITNESSES:

- AT TORIN Y Patented May 29, 1934 UNITED- STATES,

PATENT OFFICE ELECTRIC AMPLIFIER George L. Beers, East Pittsburgh, Pa.,assignor to Westinghouse Electric & Manufacturing Cominvention relatesto electric amplifiers, and

I 'it has particular relation to such amplifiers utilized in connectionwith the reception of radio signals and signals transmitted over wirelines by means of carrier currents;

The reception of radio signals, and of signals transmitted by carriercurrents over line wires, popularly known as wired-wireless, is oftenaffected by the phenomenon known as fading. The fading may be caused byfluctuations in the attenuation of the signal in the transmittingmedium, or, as it is sometimes stated, it may be caused by variations inthe height of the so-called Heaviside layer; Irrespective of the actualcause of fading, however, it results in a very annoying variation in theamplitude of the ultimate sound output of a receiving set, at timescausing the signal to decrease almost below audition, and at other timescausing the loud speaker to be overloaded.

It is accordingly one object of my invention to provide, in anamplifying system, instrumentalities and circuit connections therefor,whereby fluctuations in the amplitude of an incoming'signal shall becompensated.

Another object of my invention is to provide an automatic volume controlfor an amplifier that shall compensate variations in the inputvoltage toany stage thereof.

Another object of my invention is to provide, in'radio receivingapparatus, automatic. means for limiting the effect of a powerful localstation on the ,loud speaker, or other sound-producing instrumentality,if the apparatus is inadvertently tuned to the carrier frequency of saidlocal station during the operation of tuning to receive distantstations.

Another, and more specific, object of my invention is to provide, in anamplifier system, means for changing the volume control thereof fromautomatic actuation to manual actuation.

To attain the aforementioned objects, I provide in one embodiment of myinvention, an amplifier with an auxiliary volume-control device in thenature of a thermionic tube having its grid so negatively biased that itfunctions as a detector. The plate, or output, circuit of. the controldevice comprises .a resistor having a value in the neighborhood of .5megohm shunted by a condenser of about 4 microfarads capacity, and thesaid resistor is included in the input, or grid circuit of thetubes'that are to be controlled.

-The resistor and the condenser form a network having such a timeconstant that the audio-frequency component of the detector .tube outputdevelops no voltage across it, such voltage being developed only bycomponents below audibility, or, in other words, componentscorresponding to the usual rate of fading. The fading rate, in

general, varies from one cycle in five minutes to approximately fivecycles per minute. A connection is provided between an appropriate pointin the amplifier circuits and the grid of the control tube, whereby aportion of the potential of an incoming signal may be impressed thereon.The

changes 111 the resistance, or IR drop in the output resistor, caused bychanges in the average amplitude of the signal, are, by suitableconnections, applied to the grid of the amplifier tube controlledthereby, or to the grids of a plurality of amplifier tubes, in orderthat asignal which is increasing in strength will meet an amplifierhaving less effective amplification than normal, and a signal which isdecreasing in strength will meet an amplifier having more effectiveamplifi- 7 the control-device may be provided with appropriate biasingmeans to fix its normal potential with reference to the potentialapplied thereto from an incoming signal. If, for example, the voltagefor the actuation of the control-device,

is derived from the input circuit of a power-tube used as a seconddetector, in which the voltage for a desired output level should not begreater than approximately thirty volts, I have found that good resultsare obtained by biasing the control-device grid to a potential ofapproximately thirty volts. Specifically, in the case just re-- ferredto, the bias applied to the control-device grid would be thirty voltsmore negative than the cutoff potential, thus preventing the saidcontroldevice from functioning until the voltage on the For higheroutput levels the voltage on the the cutoff potential, the increase inthe negative direction being approximately equal to the voltage on thesecond detector grid which gives rise to the output level desired.

Such an arrangement-permits the use of a relatively low potential ofapproximately ninety REISSUi-Eil volts on the plate of thecontrol-device, and a tube analogous to the well known Radiotron 201-embodiment of my invention, illustrating a num-.

ber of ways in which it may be applied to any desired stage of anamplifier.

Fig. 2 is a diagrammatic view of a form of my invention that maybeutilized under certain conditions, as, for example, when it is desiredto control a thermionic tube handling a' considerable amount of power,or where the voltage availablefor control purposes is insumcient.

Fig. 3 is a diagrammatic view of a complete receiving system of thesuperheterodyne type, illustrating the application thereto of aprefen'ed form of my invention, and

Fig. 4 is a diagrammatic view of a modified form of my invention,wherein the control-voltage is obtained directly from the plate circuitof the second detector-tube in a superheterodyne receiving system.

Figure 5 is a diagrammatic view of my invention showing the automaticvolume control as applied to amplifiers preceding and following the tubefrom which the controlling potentials are derived.

My invention is applicable to substantially any stage of a radiofrequency amplifier, or substantially any stage of an intermediatefrequency amplifier, such as is found in receiving sets of thesuperheterodyne type. It is also feasible to derive the controlpotentials for my device from any one of the above-referred to stages.For this reason, in order to simplify the explanation of my invention, Ihave shown-my invention in Fig. 1 associated with appropriate circuitconnections for applying it to any desired amplifier stage. In theactual practice of my invention, the majority of the connections shownin Fig. 1 are eliminated, only those being retained which serve tocontrol the appropriate stage, or stages.

Referring specifically to Fig. 1, a thermionic device 1 having afilament 2, a grid 3 and a plate 4 is comprised in an amplifying stage.The input circuit for the thermionic device comprises an inductor 5,shunted by a tuning condenser 6. The filament 2 is energized from abattery '7 and plate potential is supplied from battery 8, connectedinseries with an inductor 10 shunted by a tuning condenser 11, a commonconnection of the inductor and condenser being connected. to the plate 4of the thermionic device. An inductor 12 havinga condenser 13 connect edin shunt thereto is coupled to the output inductor 10 "and is comprisedin the input circuit of a succeeding amplifier stage.

An inductor 14 having a tuning condenser 1 connected in shunt thereto iscoupled to the input inductor 5, the inductor 14 being comprised in theoutput circuit of an antenna or a preceding amplifier stage. AC-battery16 for appropriately biasing the grid has its positive pole connectedto'the filament 2 and functions in a manner which will later bedescribed in detail. A switch arm 20 cooperating with a plurality thecondenser 43, when the control is of fixed contacts 21, 22, 23, 24, 25,26, 2'7, 28, and 30 is provided. The contact 21 may be connected to anyappropriate point in the stages of the amplifier preceding thethermionic device 1 by means of a conductor 31. The fixed contact 22 isconnected by a conductor 32 to the output circuit which comprises theinductor 14 and the tuning condenser 15. The fixed contacts 23, 24 and25 are connected to separated points on the input inductor 5 by means ofconductors 33, 34 and 35 respectively. The fixed contact 26 is connectedby means of a conductor 36 to a movable arm 3'7, which makes contactwith a resistor 38 connected in shunt to the input inductor 5. The fixedcontact 2'7 is connected to the plate of the thermionic tube 1 by meansof a conductor 40 or may be connected to an intermediate point on theinductor 10. The fixed contact 28 is connected in a similar manner tothe input inductor 12 by a conductor 41, or it may be connected to anintermediate point thereon. A conductor 42 is connected to the fixedcontact 30 and extends to an appropriate point in a stage of theamplifier succeeding the thermionic device 1.

One end of the input inductor 5 is connected through a bypass condenser43 and a movable contact 44 to any desired point on the C-battery 16,whereby the initial sensitivity of the receiver may be controlled. 7

A control device 45, preferably a thermionic tube having a filament 46,a grid 47, and a plate 7 The positive poleof a C-battery 54, isconnected to the filament and is adjustably connected to the switch arm20, through a movable contact device 55, a variable resistor 56, acondenser 57 and a conductor 58.

The grid 47 is adjustably connected to the res'stor 56 by means of acontact device 60.

The end of the resistor 51 which is connected to the filament 46 of thecontrol tube, is also connected by means of a conductor 61 to a switcharm 62 with which are associated a plurality of fixed contacts 63, 64and 65. The fixed contact 64 is connected by means of a conductor 66 'tothe contact device 44 associated with the C- battery 16. The fixedcontact 63 is connected by a conductor 6'7 to an appropriate point inthe grid or input circuit of a preceding amplifying stage. The fixedcontact 65 is connected by a conductor 68 'to an appropriate pointin theinput or grid circuit of a succeeding amplifier stage.

A movable device '70 is associated with the re-- sistor 51 and isconnected by means of a conductor '71 to a movable switch arm '72 withwhich isassociated a plurality of fixed contacts '73, '74 and '75. Thefixed cbntact '74 is con nected by a .conductor '76 to the lower end ofthe input inductor 5. The fixed contact '73 is connected by a conduct r'77 to an appropriate point in the input clrcui o a preceding amplifierstage, while the fixed co tact '75 is connected by a conductor '78 to anappropriate po'nt in a succeeding amplifier stage. The switch arms 62and '72 may be linked together for simultaneous actuation as shown by adotted line '79 and a switch 80 may be provided to short-circuit appliedto different amplifierstage.

In order that the operation of the simple form of my inventionillustrated in Fig. 1 may be made clear, it is assumed that the switcharm 62 is associated with the fixed contact 64 and that the switch arm72.is associated with the fixed contact v Ii. When in this position, avariable portion of the resistor 51 is connected in shunt to the bypasscondenser 43 and is; accordingly, included in the input circuit of thethermionic device 1. It is, of course, obvious that by propermanipulation of the linked switch arms, the said resistor may beassociated with the input circuit of either a preceding or a succeeding.

amplifier stage, or by a slight modification with both preceding andsucceeding stages in which event the condensers 81 and 82 are analogousto the bypass condenser 43, which may be shortcircuited as describedpreviously.

Figure 5 illustrates the modification discussed in the precedingparagraph wherein the volume control eifect is applied to preceding andsucceeding stages with respect to the tube from which the controlpotentials are obtained. The circuit illustrated has been based on thecircuit of Fig. l, and similar reference numerals apply to correspondingand equivalent elements in both circuits. It is understood, ofcoursefthat the same modification may be embodied in any of the othercircuits illustrated in the remaining figures of my drawings. It will beapparent from an inspection of the circuit of Fig. 5 that the volumecontrol biasing potential derived from then'esistor 51 inthe platecircuit of the volume control tube is applied simultaneously to anamplifier 1'74 preceding the tube from which the control potentials havebeen derived and to an amplifier 1'75 following the tube from which thecontrol potentials have been derived.

Although the basic principles underlying the action of'the control-tuberemain the same irrespective to which portion of the receiving systemthe control-voltage is applied, it is interesting to note at this pointthat the results obtained by applying the voltage to a tube ahead of thetube from which the voltage forthe control-tube is derived diifer fromthe results obtained when the control is applied to a tube later in theseries. In the first case, the ultimate audio-output of the system willnot be reduced below a certain predetermined level, no matter how strongthe incoming signal may be, while in the second case the variouspotentials may be soadjusted that a strong signal from a local stationwill not actuate the loud speaker if the circuits of the system aretuned exactly to the carrier-wave of such station, while weaker signalsfrom distant stations may be received at normal volume.

It is also assumed for purposes of explanation, that the switch arm 20is resting upon the fixed contact 23 as indicated by the dotted lines.When in this position, the resistor 56 associated with the control tube45 is connected through the condenser 5'7 to a point on the'amplifierstage including the thermionic device 1 which assumes the highest radiofrequency potentials.

The switch arm 20 may obviously be so rotated to'associate the resistor56 with the input circuit of any desired amplifier stage, or with anappropriate point on such circuit, as indicated by the connections tothe input inductor and the connection to the potentiometer 38.

When it is desired to receive signals, the fila ments of the tubesl and45 are energized and the voltage of the C-battery 16 adjusted in orderthat the grid 3 of the amplifying. tube 1 may be maintained at the bestD. C. potential for eflicient radio frequency amplification. The bias onthe grid of the control tube 45 is adjusted to substantially the cutoffpoint, i. e., to the potential at which the tube is practicallynon-conductive. This point is preferably chosen at the lower or negativeend of the characteristic curve of the tube. When so biased, the controltube functions as a detector, an incoming signal causing an increasedfiow of current in the output circuit thereof, which includes theresistor 51.

If desired, the C-battery associated with the amplifier tube may beeliminated, in which case the tube is operated at what is known as zerogrid-potential.

By suitably adjusting the normal grid bias on the amplifier tube 1, andby properly varying the amount of the resistor 51 connected in shunt tothe condenser 43, the amplifying stage may be placed in condition toemciently amplify incoming signals of normal average amplitude.

If the average input voltage applied to the amplifier tube tends toincrease, because of any variable factor either in the transmittingmedium or in the receiving apparatus itself, there is a resultingtendency for the sound reproducer to become overloaded. The sameincreased voltage applied to the grid of the control tube tends to causean increase in the average current flowing through the resistor 51. Thisincrease in the average current causes a greater IE, or resistance, dropin the resistor 51 and the point to which the switch arm 72 is connectedconsequently assumes a more negative potential with respect to the pointconnected to the switch arm 62 than it had previous to the increase insignal amplitude.

The grid 3 of the amplifier tube 1 being connected to the said point onthe resistor is therefore caused to assume a more negative averagepotential than before and the effective amplification of the tubeconsequently decreases. It is thus apparent that when an incoming signaltion of a more negative potential to the grid of the thermionic devicewith reference to the .filament thereof. 1

It has also been found feasible to so arrange the device that a signalof an amplitude which causes the desired sound output from the ultimatestages of the amplifier is just sufiicient in amplitude to cause enoughdrop through the resistor to keep the thermionic amplifier tube biasedto an intermediate point on its effective amplification curve. Adecrease, therefore, in signal amplitude will result in a more positivebias being applied to the amplifier stage and consequently an increasein the effective amplification thereof, while an increase in theamplitude of an incoming signal will so bias the said stage, as waspreviously explained, as to decrease its effective amplification.

It is also possible to reverse the connections to the resistor 51 shownin Fig. 1, so arranging the system that an increase in amplitude of theincoming signal causes the grid of the amplifying tube to swingsufiiciently positive with respect to the filament thereof to obtainpractically the same results as if the grid were caused to swing morenegative. i

In certain receiving systems, the strength of signal derived from theamplifier is insufficient to properly actuate the control tube. In suchcase, a high frequency amplifying stage may be interposed between theamplifier-proper and the control tube or the system maybe arranged in amanner analogous to that illustrated in Fig. 2.

In the latter figure, the control tube with its associated batteries,circuit connections, etc., is shown as being replaced by adetector-amplifier system. The elements of Fig. 2 which are analogous tothose of Fig. 1 are similarly numbered.

The output resistance 51 instead of beingdirectly connected to thelinked switch arms 62 and 72 is connected between the grid 90 and thefilament 91 of a second thermionic device 92, which is supplied withfilament power from a'separate source 93. The output circuit of thethermionic device 92 comprises a source of potential as and a resistor95 analogous to the resistor 51 shown in Fig. 1. Witkthis resistor areassociated the switch arms 62 and 72 together with the cooperating fixedcontacts.

The operation of the modification shown in Fig. 2 may be explained byassuming that the incoming signal which is tending to increase, or isincreasing, in amplitude, is applied to the grid 47 of the gCOIltI'Oltube through the conductor 58 and the resistor 56. The control tube,being biased to substantially the cut-ofi point, functions as adetector, in a manner well-known to those skilled in the art. Theincoming signal, therefore, causes an increase in the current flowing inthe output circuit thereof including the resistor 51, causing the end ofthe resistor 51 associated with the C-battery as to become more negativewith respect to the point on the resistor connected to the grid 90 thanit was previously to the signal increase. sister 95 included in theoutput circuit of the tube 92, consequently increases, and the switcharm 72, which is connected to the grid of the appropriate amplifierstage, becomes more negative with respect to the switch arm 62,connected to the filament of the same stage, than it was previous tothesignal amplitude increase. The tube 92 may be of considerably higherpower than the control tube 45, the IR or resistance, drop in theresistor 95 may thus be made considerably greater than the drop in theanalogous resistor 51 shown in Fig. 1.

The system illustrated in Fig. 2 has provedto be of considerableadvantage in a receiving set of the type in which the high-frequencyvoltage developed is insufiicient to properly actuate the system shownin Fig. 1.

My invention is capable of being applied to substantially anyradio-receiving system known to the art at present, including systems inwhich equi-potential cathode tubes. are utilized. To illustrate however,the numerous variations in form which my invention may take when appliedto a variety of different radio receiving systems would necessitate alarge number of drawings and would require a great deal of repetitiousexplanation. I have, therefore, chosen for purposes of furtherdescription, the application of a preferred form of my invention to areceiving system of the superheterodyne type, in which a common voltagesourse is utilized tosupply-plate potential and grid bias to both-theamplifier tubes and the control tube.

Referring specifically to Fig. 3, a radio receiving set of thesuperheterodyne type comprising a radio-frequency amplifying stage 100,a first detector 101, an intermediate, or beat, frequency amplifyingstage 102, a second detector 103, an

audio frequency amplifying stage 104 and an oscillation generator 105,is illustrated. Each of the stages shown, with the exceptionof the firstname The current in the re-.

and second detector could, in practice, comprise a plurality of tubes,but in order to simplify the diagram the amplifying stages areillustrated as comprising only a single tube each.

, Inasmuch as the operation of the control tube is greatly facilitatedbythe utilization of relatively high input-voltages, it is desirable toprovide a source of such voltages in the receiving system. Perhaps themost convenient method is to employ as a second detector a tube of thepower-type, capable of handling an input voltage swing of as much asthirty volts, and delivering sufficient power. to enable the discardingof one of the two audio-frequency amplification stages employedpreviousto my invention. In addition to permitting the building up of voltagessumciently high to satisfactorily operate the control-device, the powerdetector gives an output of sufficient amplitude to enable a singlestage of power audio-frequency. amplification to actuate,

at maximum efiiciency, all sound reproducing devices that previouslyrequired the use of several stages of audio-frequency amplification.

The input circuit of the first radio-frequency amplifying device maycomprise a loop 106 tuned by a condenser 107. The stage may beneutralized, if desired, according to the teachings of the Rice PatentNo. 1,334,118,.by means of a condenser 108 connected between the outputcircuit thereof and one end of the loop.

Plate potential for the radio frequency amplifier 100, the firstdetector 101 and'the intermediate frequency amplifier 102 is suppliedthrough a conductor 110 which is connected to an appropriate point on avoltage source 111. Plate potential for the second detector 103 issupplied through a conductor 112 from a different point on the voltagesource, and plate potential for the audio amplifier is supplied from astill different point on the same source, a loud speaker 113 beinginterposedinthe last named connection.

The filaments of all ofthe tubes included in the amplifier proper aresupplied from the same source (not shown) one terminal of which isconnected to a common grounded conductor 114. The grounded conductor 114is connected to an intermediate point on the voltage source 111, thepoint of connection being assumed tobe at zero potential.

The grids of the thermionic devices comprised in the radio frequencyamplifying stages and the intermediate frequency amplifying stages areconnected by means of conductors 115 and 116 respectively to a secondconductor 117 which term'nates in a movable contact device 118associated with a resistor 127, analogous in some respects to theresistor 51 shown in Fig. 1.

Grid biasing potential for the first detector tube is supplied from apoint on the voltage source 111 which is approximately 9 volts negativewith respect to the filament of the tube. Grid biasing potential for thesecond detector tube, which is of the power type, is supplied through aconductor 120from a point on the voltage source 111 still more negativewith respect to the filaments, and the audio frequency power-amplifierstage is supplied with grid biasing potential through a conductor 121from a point more negative than the point which sup plies the seconddetector grid bias.

to the control tube 45 shown in Fig. 1, has a filament 123, a grid-124,and av plate 125, and is provided with a grid, or input, circuitcomprising A control tube 122, analogous in many respects- 25 mentthereof.

grid circuit of the control tube.

to the plate 125 and the other end of which is.

connected to the zero, or grounded, point on the voltage source- 111bymeans of a conductor 128. The filament of the control tube isconnected by a conductor 130 to a point on the voltage source which is90 volts negative with respect to the zero point. This method ofconnection serves to impress a positive potential of 90 volts upon theplate of the control tube with respect to the filament thereof throughthe resistor 127.

The grid 124 of the control tube is connected to one end of the resistor126, and the opposite end of the said resistor is connected to thefilament 123 through a by-pass condenser 131, and is also connected by aconductor 132 to 'a point on the common voltage source which is 103volts negative with respectto the zero, or ground point,

thereon.

The grid of the control tube, accordingly, is normally biased to apotential approximately thirteen volts negative with respect to thefila- A conductor 129 is connected between the plate 125 and a point onthe voltage source 111 approximately 20 volts negative with respect tothe zero point, and interposed in this conductor is a switch device 133.A connection 134 extends between the input resistor 126 and the grid ofthe second detector tube, a coupling condenser 135 being interposedtherein to prevent shortcircuiting a portion of the voltage source 111.

The normal operation of this modification is substantially the same asthe operation of that modification of my invention shown in Fig. 1, ifthe switch 133 is open. The grids of the thermionic devices in the radiofrequency amplifier stage and the grids of the thermionic devices in theintermediate frequency amplifier stage are connected to a point on theoutput resistor 127 by means of the contact device 118. The lower end ofthe output resistor is connected by the con: ductors 128 to thefilaments of these tubes. Consequently, a variable portion of theresistor'127 may be included between the grids and the filaments of theradio frequency and. intermediate frequency amplifier tubes and thepotential of the said grids with respect to the said filaments is thusdetermined by the IR or resistance drop in the said resistor. I Y

By reason of the fact that the grid of the control tube is normallygiven a negative bias of approximately 13 volts with respect to thefilament thereof, the tube conducts practically no current in theabsence of an incoming signal. The amount of this current can, ofcourse, be regulated by regulating the plate potential as well as thegrid biasing potential.

When an incoming signal is impressed on the second detector tube, it isalso impressed on the An increase in the average amplitude of suchsignal causes an increase in the output current of the control tubewhich flows in the output resistor thus serving to give to the movablearm 118 a more negative potential with respect to the conductors 128 and114 than it had previous to such amplitude-increase. Inasmuch as thismovable arm is connected to the grids of the radio frequency andintermediate frequency amplifier tubes and the conductor 114 isconnected to the filaments thereof, the said grids will accordinglyacquire a more neg'ativebias with respect to the filaments when thesignal amplitude increases.

The system illustrated in Fig. 3, in which are utilized three-electrodethermionic devices of the usual and well-known type while perfectlyoperable to produce the desired result, has the slight disadvantage ofnecessitating two separate sources of filament power, one for thereceiving .apparatus proper and another for the control tube. In orderto overcome this diificulty, I may also use thermionic devices of theequipotential cathode type, such, for example, as those shown in thepatent to Nicolson, 1,459,412. In this modification of my invention (notshown) the heater elements of all of the tubes may be fed from a singlesecondary winding, inasmuch as the tubes may be so manufactured that thesaid elements are thoroughly insulated from the cathodes. The cathodesof all of the tubes may be connected to the same points in the circuitnetwork that the filaments are shown as connected to in Fig. 3, no otheralterations being necessaryto change the 1system from direct toalternating current operaion.

If it is desired to substitute manual for automatic volume control, theswitch device 133 is closed and the resistor 127 is thereby connected inshunt to that portion of the voltage source 111 included betweenconductors 128 and 129.. By manually adjusting the. movable element 118along the resistor, the grids of the radio frequency amplifier tubes andthe grids of the intermediate frequency amplifier tubes may be given apotential varying from zero to the potential of the point on the voltagesource to which the conductor 129 is connected, which point has apotential of minus 20 volts.

- In the event that economy of construction is a major consideration,rather than maximum efficiency, the control-tube 122 and itsassociatedcircuits may be omitted, and the control-voltage derived directly from aresistor positioned in series in the plate circuit' of the seconddetector tube. ing the, cathode of the control tube at a high negativepotential with respect to the cathodes of the tubes controlled therebyhas been shown, if the control-tube is replaced by the seconddetector-tube, such tube must either be supplied from a separateA-battery, or must be of the equipo-' tential, or heater-cathode, type.If of the latter type, the heater-winding, or filament, may be suppliedwith energy from the same A-battery that is utilized for the balance ofthe tubes, since the cathode is thoroughly insulated therefrom.

In order. that the aforementioned modification of my invention may bemade clear, reference should now be made to Fig. 4, which is adiagrammatic view of a portion of a superheterodyne receiving systemutilizing a second .detector tube 150 of the equipotential-type. Thedetector tube 150 comprises a heating winding 151, an encircling,thermionically active cathode 152, a grid 153 and a plate 154.Anjntermediate-frequency amplifier-tube 155 of the hot-cathode type isassociated therewith by suitable circuit connections to be described indetail later, and comprises the usual filament 156, grid 157 and plate158. A single A-battery 160, or other power source, supplies cathodeenergy for both the second detector tube, and the remaining amplifiertubes of th system, including the tube 155. I

A single'source 161 of plate and grid-biasing potential is provided, toan intermediate point 162 on whiclris connected the filaments of the am-Inasmuch as the necessity for maintain plifier-tubes by means ofaconductor 163 having a grounded connection 164. The, point 162 on theplate potential source is assumed to be at .zero potential, and is thereference point from which the various potentials applied to theamplifier tubes are measured.

The cathode 152 of the detector-tube 150 is connected by a conductor 165to a point 166 on the source 161 which. has a negative potential ofapproximately 220 volts, and the grid 153 of the same tube is connectedto a point 168 by a conductor 167, having a negative potential ofapproximately 250 volts, thus placing a negative bias of approximatelyvolts on the grid. The plate 154 is connected to a point 169 having anegative potential of about 9 volts, thus impressing a positivepotential of about 211 volts thereon with respect to the cathode 152.

A resistor 1'70 is interposed in the plate circuit of the seconddetector-tube, and the grid 157 of the amplifier tube is connected to anappropriate point 171 thereon by a conductor 1'72. Inasmuch as theresistor 170 connects to a point on the source 161 which has a negativepotential 9 volts with respect to the cathode of the amplifier tube, thegrid of the latter tube is normally negatively biased to that extent. Insome instances it may be desirable to positively bias the grid of theamplifier tube with respect to its cathode, and in such event theresistor is connected to a point on the source 161 which is positivewith the respect to the point 162. All of the voltages specifled areapproximate, and they will, of course,

vary according to the type of tubes utilized.

The operation of the modification of my inventionjust described issubstantially the same as the ,operation of the modification illustratedin Fig.3. Variations in the plate current of the detector-tube give riseto variations in the resistance voltage drop in the resistor 1'70 andcause the potential of the amplifier tube grid to be so changed withrespect to the filament thereof that the effective amplification in saidtube is altered. For example, if an incoming signal, by reason orfading-in, or from any other cause, gradually increases in strength, theoutput of the detector tube increases proportionately, and the point onthe resistor 171 to which the grid of the amplifier tube is connectedassumes a more negative potential, and the effective amplificationtherein is reduced. If the incoming signal "fades-out, or diminishes instrength, the plate current of the detector tube is decreased, and thegrid of the amplifier tube becomes more positive, thus increasing theeffective amplification therein.

The principal advantage of my invention lies in the fact that it permitsa multi-tube amplifier to operate with a substantially evensound-output, irrespective of the fading or changing in averageamplitude of an incoming signal. This condition is very deslnable inconnection with receiving apparatus utilizing power amplifier tubes,inasmuch as the effect of varying signal amplitude is greatlyaccentuated thereby.

My invention has a further advantage in that it prevents the poweramplifier tubes from being overloaded and consequently, injuring thesound reproducer.

Although. I have illustrated and described only a few-specificembodiments of my invention, numerous modifications thereof will beapparent to those skilled in the art. My invention, therefore, is not tobe limited except insofar'as is necessitated by the prior art or by thespirit of the ap pended claims.

I claim as my invention:

' 1. In combinationfan amplifier comprising an electron tube device, asecond tube device having a resistor in the output circuit thereof,means for supplying anode potential to said second device, continuouslyvariable means for negatively biasing the grid of said second device tofix the output level of the amplifier, means for impressing signals onboth of said electron tube devices, and means connecting said resistorwith the grid of said first-mentioned triode whereby a tendency towardan increase in average signal amplitude results in a tendency for saidgrid to assume a more negative potential than it possessed previous tosaid amplitude-increase tendency.

2. In an amplifier, manually operable means for controlling the volumeof the output therefrom, automatic means for controlling the volumeirrespective of the amplitude of incoming signals, both of said meanshaving elements in common, and manually operable means for selecting thetype of control desirable.

3. In combination, an amplifier comprising a thermionic device arrangedto amplify high frequency alternating currents, an electric dischargedevice connected with said thermionic device to control the effectiveamplification therein, means for impressing the high frequencyalternating potential on said discharge device, and continuouslyvariable means for rendering said discharge device responsive topotentials of any predetermined amplitude, whereby electric dischargedevices of difiering characteristics may be utilized and the outputlevel of the amplifier may be preset at any desired value.

d. In combination, an amplifier comprising a thermionic device arrangedto amplify high frequency alternating currents, a second thermionicdevice having a grid and being connected with said first mentioneddevice to control the effective amplification therein, and continuouslyvariable means whereby the potential of the grid of the secondthermionic device may be adjusted relative to the filament thereof.

5. An amplifier having grid and anode electrodes, means for impressingsignals on said amplifier, means responsive to variations in the averageamplitude of said signals for controlling the amplification of saidamplifier, said last means, comprising an electric discharge devicehaving grid and anode electrodes, and common potential supply meanselectrically connected to said anodes and at least one of said gridelectrodes for supplying said amplifier and control means with anode andbias potentials.

6. An amplifier comprising a plurality of thermionic devices having gridand anode-electrodes, a thermionic device of the equipotential type forcontrolling the amplification of said amplifier, said thermionic devicehaving grid and anode electrodes, and potential supply means common tosaid anodes and at least one of said grids for supplying anode and gridpotentials to said amplifier and control device. a

7. An amplifier comprising a plurality of thermionic devices, means forimpressing signals successively on said devices, signal-responsive meanscomprising an electron discharge device for controlling the effectiveamplification in certain of said devices and potential supply meanscommon to said am lifier and said control means for supplying thethermionic amplifying devices and the signal responsive means with platepotential and grid-biasing potential.

8. In combination, a. plurality of signal amplifying thermionic devicesconnected in cascade, a signal responsive thermionic device forcontrolling the efiective amplification of said amplifying devices, acommon source of grid biasing and plate potentials for said devices, andconnections whereby the cathode of said signal-responsive device ismaintained at a high negative potential relative to the cathodes of saidamplifying devices.

9. In combination, a plurality of signal-amplifying thermionic devicesconnected in cascade, a signal-responsive thermionic device forcontrolling the effective amplification of said amplifying devices, acommon source of grid biasing and plate potentials for all of saiddevices, and connections whereby the cathode of said signal-responsivethermionic device is maintained at a high negative potential relative tothe cathodes of said amplifying devices and the anode of saidsignal-responsive device may also be maintained negative with respect tosaid cathodes but positive with respect to the cathode of thesignal-responsive device. I

10. In a radio receiving system, a plurality of amplifying stages incascade, means for deriving an audi-frequency current therefrom soundreproducing means, control means actuated by an incoming signal derivedfrom one of said amplifying stages, and means connecting saidcontrolmeans to one of said amplifying stages subsequent to theamplifying stage from which said signal was derived.

11. In an amplifier comprising a plurality of thermionic devicesconnected in cascade, said devices having input and output circuits,means for deriving a biasing potential from signal currents, said meansalso having input and output circuits, means for selectively couplingthe input circuit of said bias potential deriving means across the inputcircuits of said thermionic devices, and means for selectively couplingthe output circuit of said bias potential deriving means to saidthermionic devices whereby said potentials may be applied to any one ofsaid thermionic devices to control the degree of amplification therein.

12. In a radio receiving system, means for providing automaticvolume-control and means, including a portion of said first-named meansfor simultaneously removing said automatic volume control andsubstituting manual volume control therefor.

13. An amplifier having grid and anode electrodes, means for impressingsignals on said amplifier, means responsive to variations in the averageamplitude of said signals for controlling the amplification of saidamplifier, said last means comprising an electric discharge devicehaving grid and anode electrodes, and common potential supply meanselectrically connected to said anodes and said grid electrodes forsupplying said amplifier and control means with anode and biaspotentials.

14. In combination, an amplifier comprising a plurality of electrondischarge devices one of which is arranged and connected to function asa detector, a second detector device having an output circuit, means forimpressing signal voltages simultaneously on the input circuits of bothof said detector devices, said output circuit being coupled to inputcircuits of certain of said electron discharge devices preceding andfollowing said first mentioned detector whereby changes in the currentin said output circuit are effective to 11 cause alterations in the gridpotentials of said devices in said amplifier.

, GEORGE L. BEERS.

